1. Field of the Invention
This invention relates to an apparatus for effecting chemical reactions in the presence of fluidized solids. The apparatus includes at least one centrifugal separator of the cyclone type for removing entrained solids from effluent gas and vapors, a conduit or dip-leg for returning separated solid particles from the cyclone to the fluidized system, and a unidirectional valve positioned at the lower end of the dip-leg. More particularly, the invention relates to an improved apparatus which reduces erosion damage to the unidirectional valve by the fluidized solids.
2. Description of the Prior Art
Systems wherein a chemical reaction is carried out in the presence of fluidized finely divided solid particles are well known and have found wide acceptance in a variety of fields. A particularly important application involves the conversion of various feedstocks by contact with fluidized finely divided catalyst particles as, for example, in the fluidized catalytic cracking of hydrocarbon feedstocks. Alternatively, the fluidized solid particles may themselves comprise a reactant as in the fluidized combustion of finely divided particles of coke or coal. In addition, the fluidized particles can be substantially inert and serve merely as a heat transfer medium and/or a substrate upon which reaction products may deposit.
A bed of fluidized solids is produced by the upward passage of a fluidizing gas through a bed of finely divided solid particles. The upward flow of gas through the solid particles exerts an upward or suspending force on the particles and serves to lift and agitate the particles. In a vessel containing a fluidized bed of solids, the lower portion of the vessel will contain a phase of high particle density having a fairly well defined upper surface. This lower dense phase behaves in many respects like a liquid. A dilute phase or disengaging space is located above the dense phase wherein the particle density is low and a separation of solid particles from the fluidizing gas is largely completed. Particle fluidization comprises a highly effective gas-solid contacting process. The particles are maintained in a suspended, turbulent state by the fluidizing gas and migrate more or less freely to all portions of the dense bed.
Ordinarily, it is desirable to separate effluent gases from solid particles as completely as possible prior to the discharge of these gases from a vessel which contains a fluidized bed. A substantial separation of gases and solids occurs within the vessel, but small amounts of solids are entrained by the fluidizing gases, are carried into the dilute phase, and are discharged together with the gases if additional separation means are not provided. Centrifugal separators of the cyclone type have been widely used to provide such additional separation.
A cyclone separator functions by allowing the particle laden gases to enter a cylindrical or conical chamber tangentially at one or more points, and the gases are discharged through a central opening. The solid particles are forced to the walls of the separator as a consequence of centrifugal acceleration and are led to a particle return conduit or dip-leg for return to the vessel. Frequently, the dip-leg extends into the lower dense phase but can also terminate in the upper dilute phase. If necessary, improved efficiency can be achieved by passing the particle laden gases through two or more cyclone separators which are joined in series.
A unidirectional valve is frequently positioned either within the dip-leg or at its lower end. This valve serves to permit the flow of separated solids from the cyclone while preventing a reverse flow of gases and solids into the cyclone from the dip-leg. A trickle valve is a widely used unidirectional valve comprising a pivotally mounted valve closure means at the end of the dip-leg which is exposed to the fluidized solids. This exposure to the fluidized solids subjects the valve to erosion as a consequence of mechanical action by the solid particles. After a period of time, this erosion damage may be severe enough to result in a complete failure of the valve. Such failure permits a significant flow of gases and solid particles up the cyclone dip-leg. This reverse flow of solids into the cyclone will, in turn, result in an increased discharge of particulate material with the effluent gases from the cyclone. If these effluent gases are discharged directly into the atmosphere, the additional particulate content is environmentally undesirable. If, instead, the effluent gases are discharged into associated process equipment for further treatment or separation, the increased particulate content is undesirable because of its erosive effect on the gas transfer lines and the need for removal of the solids during the subsequent processing steps. In addition, when the fluidized solid is a catalyst, the loss of such solid can be undesirable for the additional reason that such catalytic materials are frequently quite costly.
U.S. Pat. No. 2,838,063 has suggested that trickle valves be shielded from below. In addition, U.S. Pat. Nos. 2,838,062 and 2,901,331 teach the use of a perforated tubular shroud or housing which encloses the trickle valve on both top and bottom. The prior art does not, however, suggest the desirability of placing a conical shroud above the trickle valve to provide protection from the downward impingement of solid particles.